In conventional home freezer systems, an ice-making machine includes at least one ice mold. However, more sophisticated systems may include a series of ice molds. In order to make ice, the ice mold is first filled with cold tap water. The water and ice mold are then cooled by heat conduction through a surface which the ice mold is placed upon. The water and ice mold are also cooled by convection through the air located above the water and the ice mold. As heat is extracted, the water is slowly converted to ice. However, this method for forming ice cubes can take an hour or more.
The above described process is too slow to provide an adequate supply of ice cubes in a restaurant or vending machine application without the use of a large freezer and several ice molds. To circumvent this problem, commercial ice makers use ice molds that are cooled directly through circulating refrigerant. Consequently, cooling capacity is delivered directly and rapidly to the ice molds. Commercial ice makers are also designed to automatically fill the ice molds with water when they are empty and to automatically empty the ice molds when they are filled with ice.
The challenges associated with automatic ice-making are several and include the following: preventing freezing in pumps and plumbing when supercooled water is circulated, achieving uniform and rapid filling of all the ice molds, achieving complete and uniform freezing in all the ice molds, achieving complete release of the ice cubes from the ice molds when freezing is complete, minimizing freezing time and energy consumption, and achieving a high yield. It is also desirable in some cases to produce ice cubes with a high degree of clarity.
When liquid water is cooled to 32.degree. F., the water begins to freeze. The freezing of the water will take place as the heat of fusion (79.7 cal/gram) is removed. During freezing a water-ice mixture is present, and the water and ice remains at a temperature of 32.degree. F. until freezing is complete, assuming there is adequate thermal contact between the water and ice. Once freezing is completed, the temperature of the ice will drop as more heat is extracted. Freezing will also begin if an ice piece or other suitable "seed" crystal is present in sub-freezing (.ltoreq.32.degree. F.) water. A seed crystal initiates ice growth starting at the surface of the seed and progressing outward. Freezing can also be initiated in sub-freezing water if the water is subjected to a sudden vibration. At low enough temperatures, a tap on the side of the container holding the sub-freezing liquid can be sufficient to initiate freezing.
Absent a seed crystal or vibration, it is possible to cool water to a temperature below 32.degree. F. Once water is cooled below its freezing point, i.e., 32.degree. F., it is considered to be supercooled. Supercooled water will rapidly begin to freeze when exposed to a "seed" crystal, sharp vibration or small vibrations at extremely low temperatures. Due to the 79.7 cal/gram heat of fusion, it is possible for a given mass of supercooled water to have more heat content than the same mass of ice at 32.degree. F. For instance, the heat content of 10 grams of 8.degree. F. liquid water is 2166 cal while the heat content of 10 grams of 32.degree. F. ice is 1502 cal. There is considerably more heat (44% more) in the liquid water than in the ice. Yet, the water is at a lower temperature than the ice. In order for the 8.degree. F. water to freeze entirely, its extra 664 cal (2,166-1,502) of heat content would have to be rejected to ambient air.
If approximately 16.7% of the 8.degree. F. water were converted to ice at 32.degree. F. and approximately 83.3% was to remain in a liquid state at 32.degree. F., the heat content would be 2166 cal which is the same heat content as the original 8.degree. F. water. This is essentially what happens once freezing is initiated in supercooled water. A volume of a gallon or more of supercooled water at a sub-freezing temperature will convert to a slush (small ice particles+water) in a matter of seconds once freezing has been initiated. When the supercooling is eliminated through freezing, the freezing stops. The ratio of ice to liquid is dependent on the degree of supercooling in the liquid water before the formation of ice has occurred.
FIG. 1 illustrates the fraction of liquid water in a slush mixture, following its formation from supercooled water, as a function of the initial temperature of the supercooled water. As can be seen, 27.degree. F. water can be expected to form a slush mixture of 97% liquid water and 3% ice. Similarly, -20.degree. F. water will form a slush mixture of 64% liquid water and 36% ice. Also, note that -111.degree. F. water will form solid ice.
An automatic ice-making system typically has some degree of plumbing associated therewith to properly route the water. Some systems may also include pumps and automatic valves as well. In these systems, there is no problem associated with supercooled water as long as it is completely liquid. However, when and if the supercooled water converts to a slush, the small ice particles in the slush can cause clogging in the plumbing, the pump and/or the valves as well as cause ice accumulation in undesired locations. To overcome these problems, some known systems prevent or minimize supercooling at undesired locations by adding tap water to the system or by utilizing heaters. This results in system cooling inefficiencies as more water is cooled or water is both cooled and heated. Ideally, a system will utilize most of its cooling capacity in forming ice. In systems that have supercooling, efficiency will be maximized by converting the supercooled water to ice without adding heat to it first.
The known prior art includes U.S. Pat. No. 4,671,077, issued to Paradis, which describes a system in which water is deliberately supercooled to increase the capacity of a heat exchanger. Water having a temperature of 32.degree. F. or warmer enters the heat exchanger and exits as supercooled water. The supercooled water is then deliberately used to make slush in a reservoir rather than on the surface of the heat exchanger itself. Part of the supercooled liquid water flowing from the heat exchanger is transformed to ice upon contact with the water in the reservoir and is used for space cooling. Alternatively, the ice obtained by this process may be filtered for various other applications, such as soft ice for packaging and preserving fish, for the preservation of certain vegetables, and for making slush drinks.
Another problem associated with ice-making systems is the lack of clarity in the ice cubes. Two contributing factors in the lack of ice clarity include flaws from internal stresses associated with rapid ice formation and/or induced by ice expansion against the mold cavity, and the entrapment of small air bubbles as liquid water converts to ice.
The solubility of air in liquid water is greater at lower temperatures than at elevated temperatures. For instance, the solubility of air in water is 2.5 times greater at 32.degree. F. than at 203.degree. F. Any air dissolved in the water above the concentration that can be contained by the solubility of air in ice must be released when the liquid water freezes. In slow cooling processes excess dissolved air has time to be released by the water as it slowly freezes. This is not necessarily the case in a more rapid freezing process as is found in automatic ice-making machines equipped with directly cooled ice molds. Similarly, in cases of rapid ice formation, internal strains can be associated with the forming of ice as it expands if it is unable to expand against the ice mold.
Clarity of the ice cubes can be improved by driving off trapped air before the water reaches the ice molds. However, heating the water with a heater or using hot tap water when the system is filled to eliminate trapped air has the disadvantage of adding heat to the system, and thereby lowering system efficiency.
A further problem associated with ice-making systems is the difficulty associated with achieving uniform and rapid filling of the ice mold and freezing in the ice mold. The use of a fine spray of water onto a chilled ice mold has been contemplated as can be seen, for example, in U.S. Pat. No. 4,510,144, issued to Nelson, and U.S. Pat. No. 3,908,390, issued to Dickson et al. However, excess or make-up water is abundant resulting in an inefficient system due to a loss in cooling capacity as the excess water is recirculated.